Process for polymerizing tetraoxane

ABSTRACT

Tetraoxane is polymerized in the presence of both a carboxylic anhydride and a specific polymerization initiator, to obtain an oxymethylene polymer having excellent heat-stability. When thiirane or its derivatives, thiocyanates, or isothiocyanates are employed as the polymerization initiator, the thermal stability of the resulting polymer is further improved. An ionizing radiation or an ultraviolet light can concomitantly be employed before and/or in the course of polymerization.

United States Patent 1 Yoshida et al.

[ 1 March 6, 1973 PROCESS FOR POLYMERIZING TETRAOXANE [75] lnventors:Masaru Yoshida; Yoshiaki Nakase;

Akihiko Ito, all of Takasaki, Japan [73] Assignee: Japan Atomic EnergyResearch Institute, Tokyo, Japan [22] Filed: May 18, 1971 211 Appl.No.:144,636

[30] Foreign Application Priority Data May 20, 1970 Japan ..45/4238lJuly 4, 1970 Japan ..45/58l43 [52] US. Cl. ..260/67 FP, 204/159.2l [51]Int. Cl ..C08f l/l8, C08g l/OO, C08g 11/00 [58] Field of Search ..260/67FP; 204/l59.21

[56] References Cited UNITED STATES PATENTS 3,l93,532 Sidi ..260/67 FP3,457,226 7/l969 Miyake et al. ..260/67 FP 3,553,090 l/l97l Yamashina etal.. .....204/159.2l 3,652,436 3/1972 Nakase et a1 ..204/l59.21

Primary Examiner-William H. Short Assistant ExaminerL. M. PhynesAttorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT 5 Claims, NoDrawings PROCESS FOR POLYMERIZING TETRAOXANE BACKGROUND OF THEINVENTION 1. Field of the Invention:

This invention relates to a process for preparing an oxymethylenepolymer which comprises polymerizing tetraoxane in the presence ofanhydride of an organic carboxylic acid having no ethylenic double bond,by means of a specific polymerization initiator.

2. Description of the Prior Art:

It is well known that oxymethylene polymers can be obtained bypolymerizing formaldehyde, trioxane or tetraoxane. However, the polymersthus obtained are inadequate in chemical and thermal stability and areeasily depolymerized to formaldehyde. Therefore, these polymers requiresome stabilization treatment in order that they may be commerciallypracticable. According to one conventional process, a producedoxymethylene polymer is further treated with a carboxylic acid anhydrideto be stabilized, but this process is disadvantageous in that many stepsare required for the post-treatment. In another process, formaldehyde ispolymerized in the presence of an anhydride of a carboxylic acid, butthis process is disadvantageous in that the formaldehyde monomer must behighly purified before the polymerization in order to obtain asatisfactory result.

It is know that tetraoxane is polymerized in the presence of a catalystsuch as BF (U.S. Pat. No. 3,457,226), but it is disadvantageous in thatcare must be taken in handling such very reactive catalyst; in addition,such catalyst remaining in the product polymer has to be neutralizedafter polymerization is completed.

It is also known that trioxane is polymerized in the presence of ananhydride of aliphatic carboxylic acids by means of an ionizingradiation (Japanese Pat. publication No. 26594/1963). In this process,the anhydride only serves to enhance the polymerization rate of theunpurified trioxane and the thermal stability of the resulting polymeris not improved. Japanese Pat. publication No. 20309/ 1968 discloses aprocess for copolymerizing trioxane and itaconic anhydride in thepresence of a peroxide catalystyhowever, the resultant polymer lacks theproperties required for an oxymethylene polymer. According to theprocess disclosed in Japanese Pat. publication No. 15629/ 1967, asomewhat thermally stabilized oxymethylene polymer is obtained bypolymerizing trioxane and itaconic anhydride by means of an ionizingradiation, but the process is disadvantageous in thatthe polymerizationrate is too slow. U.S. Pat. No. 3,346,663 discloses a process forpolymerizing trioxane with an anhydride of organic acids in the presenceof Lewis acids or Friedel- Crafts catalysts. However, the molecularweight of the polymer is so low that the polymer is not adequate forpractical use when an anhydride of organic acids is added to obtain apolymer having good heat stability.

The foregoing demerits in the prior art have been solved by the presentinvention, whereby thermally stabilized oxymethylene polymers areobtained with greater ease and in higher yield than when trioxane ispolymerized, by polymerizing tetraoxane in the presence of an anhydrideof a carboxylic acid by means of a specific polymerization initiator.Incidentally, it is to be noted that a process for polymerizing trioxanecan not always be applied to a process for polymerizing tetraoxane,since the reactivity of trioxane in the polymerization reaction differsfrom that of tetraoxane as described for example in U.S. Patent No.3,457,226; French Pat. No. 1,424,655; J. Polymer Sci., B6, 727 (1964)and the like.

A process for polymerizing a cyclic ether in the presence of maleicanhydride by means of an ionizing radiation or a radical initiator isdisclosed in Japanese Pat. publication No. 8829/1968, the examples ofwhich employ 3,3-bis-chloromethyl-oxetane or mixture thereof withtrioxane. However, the resultant polymers have no thermal stability. Thepresent invention is clearly distinguished from this process, at leastin the use of an anhydride of carboxylic acid having no ethylenic doublebond (excluding maleic anhydride, itaconic anhydride, etc.) and use of adifferent species of polymerization initiators. It is noted that themolecu lar weight of the resultant polymer markedly decreases and thethermal stability of the polymer is hardly enhanced, when tetraoxane ispolymerized in the presence of a small amount of maleic anhydride oritaconic anhydride as shown in the following reference examples. Fromthis fact, it is considered that the function of a carboxylic acidanhydride having no ethylenic double bond differs from that of maleicanhydride or itaconic anhydride.

SUMMARY OF THE INVENTION An object of this invention is to provide aprocess for preparing a novel oxymethylene polymer which comprisespolymerizing tetraoxane in the presence of an anhydride of a carboxylicacid having no ethylenic double bond by means of a specificpolymerization initiator. Another object of this invention is to providea novel heat-stabilized oxymethylene polymer thus obtained.

Other objects of this invention will become apparent in the followingdescriptions.

The process of this invention, if desired, can be carried out by theconcomitant use of an ionizing radiation or an ultraviolet light inadvance of and/or in the course of polymerization.

In the radiation-induced polymerization of tetraoxane no polymer wasobtained in the molten phase or in solution (J. Polymer Sci. B1, 427(1963)). However, in the case of the concomitant use, the polymerizationis also carried outin liquid state. The concomitant use brings about asynergistic effect on the rate of polymerization. In other words, therate is much higher in the case of concomitant use than the sum of therates where the initiator and radiation are applied separately.Therefore, the effect of ionizing radiation in the presence of theinitiators in this invention is essentially different from that of theknown use of radiation.

The organic carboxylic acid anhydride to be employed in this inventionis at least one species of an anhydride of a carboxylic acid having noethylenic double bond, which typically includes acetic anhydride,propionic anhydride, butyric anhydride, benzoic anhydride, succinicanhydride and phthalic anhydride etc.

The polymerization initiator to be employed in this invention isselected from iodine, bromine, a halogenohydrocarbon, sulfur, an organicsulfur compound and an electron acceptor, some of the initiators beingdefined below. Incidentally, trioxane can hardly be polymerized with theinitiator specified hereinafter.

The halogenohydrocarbon to be employed in the present invention isrepresented by the following formula;

wherein R is a radical having one to carbon atoms selected from thegroup consisting of a saturated or unsaturated aliphatic hydrocarbonresidue, a saturated or unsaturated alicyclic hydrocarbon residue and aradical in which any hydrogen atom of the residues is substituted foraryl, alkoxy, carbonyl, alkoxycarbonyl or aryloxycarbonyl radical; X isthe same or different kind of halogen atoms selected from the groupconsisting of F, Cl, Br and I; and n is a positive integer not exceeding10. Said halogenohydrocarbon is typically exemplified by methylenechloride, ethylene chloride, chloroform, carbon tetrachloride, vinylchloride, chloral, chloroacetone, chloromethyl acetate, phenylchloroacetate, hexachloroethane, benzyl chloride, tetrafluoroethylene,dibromotetrafluoroethane, bromoform, tribromoacetaldehyde, aryl bromide,methyl bromide, ethyl bromide, methylene bromide, ethylene bromide,bromal, bromopropene, bromocyclohexane, chlorocyclohexane, benzylbromide, methyl iodide, ethyl iodide, propyl iodide, isopropyl iodide,ethylene iodide, benzyl iodide, iodoform, 2- bromoethyl ethyl ether,chloromethyl ethyl ether, 2- chloroethyl ether, 3-bromocyclohexene,3-chlorocyclohexene, and 4-chlorocyclohexene.

The organic sulfur compounds to be used in this invention include thefollowing compounds. The examples of the divalent sulfur compound arethioaldehydes (e.g., trithioformaldehyde, etc.), thioketones (e.g.,methyl phenyl thioketone, etc.), thioalcohols (e.g., methyl mercaptan,ethyl mercaptan, methyl thioglycolate, etc.), thiocarbonic anhydrides(e.g., carbon disulfide, carbonyl sulfide, etc.), thiocarbonic ester(e.g., ethylene trithiocarbonate, etc.), thioethers (e.g., methylsulfide, ethyl sulfide, dimethyl disulfide, tetrahydrothiophene,thiiranes (i.e. episulfides), etc.), thioamides (e.g., acetothioamide,etc.), thiourea and its derivatives (e.g., ethylenethiourea, methylthiourea, etc.), thiocyanates (methyl thiocyanate, ethyl thiocyanate,etc.) isothiocyanates (e.g., methyl isothiocyanate, ethylisothiocyanate, etc.), alkylxanthogenates (e.g., ethylethylxanthogenate, etc.). The examples of tetravalent or hexavalentsulfur compounds are sulfates (e.g., dimethyl sulfate, diethyl sulfate,etc.), sulfites (e.g., dimethyl sulfite, diethyl sulfite, glycolsulfite, etc.), alkyl sulfones (diethyl sulfone, etc.), alkylsulfoxides(e.g., dimethyl sulfoxide, etc.), sulfonium compounds (e.g., trimethylsulfonium iodide, etc.), thiophene-derivatives-s-oxide (e.g.,dihydrothiophenel -dioxide, tetrahydrothiophenel oxide,tetrahydrothiophenel -dioxide, etc. dithiorane-derivatives-s-oxide e. g.1,3- dithioranedisulfone, 'y-sultone, etc.), oxathianderivatives-s-oxide(e.g., 8-sultone, 1,3-oxathian-l-dioxide, etc. and trithianoxides (e.g.,trimethylenetrisulfoxide, trimethylenetrisulfone, etc.

Examples of thiiranes, thiocyanates and isothiocyanates which are veryuseful for thermal stabilization of the produced polymers are shownbelow. Thiiranes include ethylene sulfide, propylene sulfide, l-butenesulfide, 2-butene sulfide, isobutene sulfide,

trimethylethylene sulfide, tetramethylethylene sulfide, cyclopentenesulfide, cyclohexene sulfide, 4-methylcyclohexene sulfide, cycloheptenesulfide, cyclooctene sulfide, styrene sulfide, 3-mercaptopropylenesulfide, 3-chloropropylene sulfide; and the thiocyanates include methylthiocyanate, ethyl thiocyanate, propyl thiocyanate, isopropylthiocyanate, butyl thiocyanate, isobutyl thiocyanate, amyl thiocyanate,isoamyl thiocyanate, allyl thiocyanate, benzyl thiocyanate, and phenylthiocyanate; and the isothiocyanates include methyl isothiocyanate,ethyl isothiocyanate, n-propyl isothiocyanate, isopropyl isothiocyanate,n-butyl isothiocyanate, benzyl isothiocyanate and phenyl isothiocyanate.

The electron acceptors to be employed in this invention typicallyinclude quinones (e.g., benzoquinone, p-

chloranil, etc. nitriles e. g. tetracyanoethylene acrylonitrile, etc.nitro-compounds (e g. tetranitroethane, nitrobenzene, m-dinitrobenzene,

l ,3 ,S-trinitrobenzene, diphenylpicrylhydrazil, etc. nitrogen-oxides(e.g., nitrous oxide, nitrogen monooxide, etc.) and sulfur hexafluoride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with thisinvention, polymerization can be carried out as follows:

At least one species each of an initiator and a carboxylic acidanhydride are added to tetraoxane. When the compound to be added israther volatile, the

polymerization system is confined in a sealed vessel. In the case ofsolid phase polymerization, the initiator and carboxylic acid anhydridecan be added to crystalline tetraoxane in the form of drops, spray orvapor, and if necessary then melt-mixed with tetraoxane and solidified.In liquid phase polymerization, they can be added to the liquid phasecontaining tetraoxane, or mixed with crystalline tetraoxane and thenmelted or dissolved into a solution. The initiator and carboxylic acidanhydride can be added to tetraoxane simultaneously or separately byvarious ways. When two or more species of polymerization initiators orcarboxylic acid anhydrides are used, the order of addition, ifnecessary, may be determined by one skilled in the art according to thenatures of the compounds to be added.

The amount of initiator to be added depends on the species thereof, butis generally about 10- 10 percent by weight, preferably about 10' 3percent by weight of tetraoxane. The amount of carboxylic acid anhydrideis about 10' 15 percent by weight, preferably about 10 5 percent byweight of tetraoxane. When the initiator and carboxylic acid anhydrideare solids at room temperature or are used in low concentration, it isadvantageous to use them in the form of a solution in a solvent inactiveto the initiator, anhydride and formaldehyde, such as benzene orn-hexane etc. The carboxylic acid anhydride can be added to thepolymerization system continuously or intermittently as it is consumedinstead of charging the total amount from the start of polymerization.

When an ionizing radiation or an ultraviolet light is concomitantlyemployed, the results are substantially the same whether irradiation iseffected before or after the polymerization initiator is added to thepolymerization system. A specific initiator such as thiiranes,thiocyanates and isothiocyanates can be irradiated with an ionizingradiation and then added to the polymerization limiting the scope of theinvention. The product of each of the examples is a white crystallinesubstance. The melting points of the polymers obtained according to thefollowing examples range from 167 to 180C. The

system. In the case of in-source polymerization, a dose 5 viscositynumber [17] is determined with respect to a rate of about l0 rad/hr isemployed. The total solution in p-chlorophenol containing 2 percentadose in post-polymerization is generally about 10 pinene at 60C. Thethermal stability of the polymer is l0rad. In the case of in-sourcepolymerization (includindicated as 222 which means average ing lightirradiation), the irradiation temperature is thermal ompo i ion ratewhen the polymer is identical with that of polymerization. In the caseof 10 heated at 2 o 1 our under nitrogen Stream. The post-polymerization(including light irradiation),the iramount of the additives is indicatedin percentage of radiation t ratur t b ithi th range i the basis of theweight of tetraoxane, unless otherwise which tetraoxane is in the solidstate. When irradiation Specifiedis carried out at a temperature higherthan 60C, insource polymerization proceeds except for the case inEXAMPLE 1 which the irradiation time is y Short; P One gram each oftetraoxane purified by sublimation polymerization can of course beeffected thereafter. Inwas put in a glass ampoule and acetic anhydrideor cidentally, the useful radiations include alpha rays, beta propionich d id d Carbon di lfid or methyl y gamma y electron beam. y neutroniodide were added thereto in the amounts as shown in beam, beams ofheavy particles and mixtures thereof. Table 1. Polymerizations werecarried out at 105C for An ultraviolet light from a mercury lamp etc.can also 2 hours. After the polymerization, white crystalline beutilized. polymers were obtained by washing with acetone so as There isno special restriction in the polymerization to remove unreactedmaterials. The polymerization temperature; however, desirable resultsare obtained results are shown in Table 1.

' TABLE 1 Polymerization conditions Results Acid anhydridePolymerization initiator Km Yield (per- \is- Amount Amount (percent/cosity Species (percent) Species (percent) cent) min.

0.1 Carbon disulfide-. 2.5 95 0.91 2.0 0.1 4.5 05 0.00 2.1 i8 i1? 328:12 8:? 5.0 o 2.5 07 0.10 0.3 5.0 Methyl iodide 4.5 100 0.08 0.2 0.1Carbon dlsulfidon 2.5 90 0.73 2.4 0.1 Methyl i0dide 4.5 05 0.80 3.0Propionic'anhydride do 5.0 2.5 07 0.12 0.3 5.0 4.5 04 0.10 0.4Noacldanhydfldeadded --l'ea;b8;;arsnra:: 3:2 ii $13 when polymerizationis carried out at a temperature of 90C or higher. The polymerization insolid state is preferable in order to give a product having the higherheat stability. Even when the temperature of the heating bath is higherthan the melting point of tetraoxane, polymerization may proceed in thesolid state during temperature raise. There is no restriction, either,with respect to the atmosphere in which the polymerization is carriedout; polymerization can be carried out either in air, vacuum or an inertgas. The term solid phase polymerization referred to herein meanspolymerization carried out when tetraoxane exists in the solid state(including dispersion), and liquid phase polymerization meanspolymerization carried out when the system containing tetraoxane is inthe liquid state. Insource polymerization means polymerization whichproceeds during irradiation by means of an ionizing radiation or anultraviolet light. Post-polymerization means polymerization whichproceeds after irradiation in the absence of the radiation source.

After polymerization, the reaction mixture is washed with a solvent suchas acetone, benzene, etc. which is a good solvent for both tetraoxaneand the additives so that the produced polymer may be separated from theunreacted materials.

Now the invention is illustrated by way of working examples. Theseexamples are included for better understanding of the invention andshould not be taken as COMPARATIVE EXAMPLE 1 One gram of trioxane wasput in a glass ampoule and thereto were added 1.0 percent aceticanhydride and 4.5 percent methyl iodide. Polymerization was carried outat 55C (the temperature being considered to be the most preferable forsolid state polymerization of trioxane) for 2 hours. No polymer wasobtained.

EXAMPLE 2 One gram each of tetraoxane purified by sublimation was put ina glass ampoule, and thereto were added a carboxylic acid anhydride andan organic solvent if desired, followed by addition of 2.5 percent ofcarbon disulfide. Polymerizations were carried out at C for 2 hours.Thereafter, the resulting polymers were washed with acetone. The resultsare shown in Table 2.

EXAMPLE3 One gram each of tetraoxane purified by sublimation was put ina glass ampoule. Thereto were added 0.5 percent of propionic anhydrideand 2.5 percent of methyl iodide.

Polymerizations were carried out at 105C and 120C for two hours, and theproducts were treated as in Example 1. The results are shown in Table 3.

TABLE 3 Polymerization Results Temperature Yield K222 (%lmin) Viscosity[1 EXAMPLE 4 One gram each of tetraoxane purified by sublimation was putin a glass ampoule. Thereto were added 0.5 percent of propionicanhydride and a polymerization initiator as given in Table 4.Polymerizations were carried out at 105C for various periods of time,and the products were treated as in Example 1. The results are One gramof tetraoxane purified by sublimation was put in a glass ampoule and 0.5percent of propionic anhydride was added thereto. The mixture wascompletely melted at 130C and 4.5 percent of methyl iodide was addedthereto, followed by liquid phase polymerization for 2 hours.Thereafter, the product was treated as in Example 1, and the polymer ofK 0.85 %/min, [1;] 1.1 was obtained in 100 percent yield.

The mixture of 1 g tetraoxane purified by sublimation and 0.5 percent ofpropionic anhydride was dissolved in 1.0 ml methylene chloride at 100C,4.5 percent methyl iodide was added thereto. Polymerization was effectedfor 2 hours, and the polymer of K 0.95 %/min, [17] 0.5 was obtained in71 percent yield.

EXAMPLE 6 One gram each of tetraoxane purified by sublimation was put ina glass ampoule, and thereto were added 0.5 percent of propionicanhydride and a polymerization initiator as given in Table 5. Theampoules were irradiated with gamma rays from Co-60 at 78C, and

then polymerization was carried out for 2 hours in :1 105C heating bath.Thereafter, the products were treated as in Example 1 in order to removeunreacted materials. The results are shown in Table 5.

10 TABLE 5 Polymerization Conditions Results Dose of initiator gammarays Yield K Viscosity Species Amount(%) (rad) (%/min) [1;]

10" 1 X 10 67 0 90 1 4 10" 1 X 10' 80 0.85 1.3 Carbon disulfide 0.5 0 820.50 1.4 0.5 1 X 10" 97 0.60 1.4 10* l X 10' 70 0.60 1.2

Methyl iodide 0.5 0 35 0.55 2.0 0.5 1 X 10 93 0.60 1.5 Not added 1 X 1040 1.15 0.8

EXAMPLE 7 One gram of tetraoxane purified by sublimation was put in aglass ampoule, and 0.01 percent of bromine and 1.0 percent of aceticanhydride were added thereto, followed by sealing the ampoule.Polymerization was carried out for 2 hours in 105C heating bath. Acrystalline polymer of [n] 1.2 and K 0.31 was obtained in 95 percentyield.

EXAMPLE 8 One gram of tetraoxane purified by sublimation was put in aglass ampoule, and 1.0 percent of propionic anhydride and 0.01 percentof 1,3,5-trinitrobenzene were added thereto. The ampoule was sealed andpolymerization was carried out for 4 hours in a 1 10C heating bath. Apolymer of [1 0.5, K 0.17 was obtained in 77 percent yield.

EXAMPLE 9 TABLE 6 Polymerization conditions Results Amount Amount DoseTemp. Time Yield Thilranes (percent) Acid anhydride (percent) (rad.) 0.)(hr.) (percent) Km [1 Ethylene sulfide 0.05 Propionlc anhydride 0. 04X10 110 2 99 0. 15 1. 8 Propylene sulfide 2. 0 Succlnic anhydride 2. 01X10 9O 6 0. 30 1. 4 Cyclohexene sulfide 0. 1 Acetic anhydride 0. 1 1X10105 2 99 0. 05 2. 4 Do 0.1 d0 1.0 1X10 105 2 99 0.06 1.5 Propylenesulfide. 0. 1 1X10 130 6 99 0. 17 1. 4

Reference examples:

Ethylene sulfide. 4X10 110 2 99 0. 24 2. 5 Propylene sulfide 1X10 90 650 0. 45 2. 5 Cyclohexene su1fide 1X10 105 2 99 0. 2O 4. 0 Aceticanhydride 1. 0 1X10 105 2 0. 95 0. 9 Succlnic anhydride 2. 0 1X10 6 330. 98 1. 3

EXAMPLE 10 Two(2) grams each of tetraoxane purified by sublimation wasput in a glass ampoule. Polymerizations were carried out under theconditions given in Table 7 in liquid phase. After polymerization, theproducts were treated as in Example 9. The results are shown in Table 7.

cept that ultraviolet light from high pressure mercury lamp (ToshibaHLS-4002 Type) was employed instead ofgamma rays. A polymer of [17] 1.7,K 0.30 was obtained in 71 percent yield.

EXAMPLE 13 One gram each of tetraoxane purified by sublimation was putin a glass ampoule, and a thiocyanate and a car- TABLE 7 Polymerizationconditions Results Proprene Propionic sulfide, anhydride, Yield Amountamount amount Dose Temp. Time (per- Solvent (percent) (percent)(percent) (rad.) C.) (hr.) cent) Km [1;]

1.0 1.0 130 2 as 0.21 1.4 Cyclohexane 100 1.0 1.0 100 2 21 0.33 1.1 Do100 1.0 1.0 1 1X10 100 1 65 0.21 1.0 1.0 130 2 99 0.70 2.5 Referenceexamples:

Cyclohexane 100 1. i 1 10 100 1 65 0. 47 1.7 Do 100 .1 1x10 100 1 1 1In-source.

EXAMPLE l 1 Ten( 10) grams of tetraoxane purified by sublimation wasirradiated with l X 10 rad gamma rays from Co-60 at room temperature,and then was put in a glass ampoule, followed by addition of L0 percentpropylene sulfide and 0.1 percent propionic anhydride. The ampoule wassealed and polymerization was carried out for 6 hours in 105C heatingbath. After the boxylic acid anhydride were added thereto as given inTable 8. The ampoules were sealed and irradiated at room temperaturewith gamma rays from Co-60 in the dose of 10 l0 rad, followed bypolymerization at 105C. After the polymerization, the products werewashed with acetone and dried under reduced pressure. White, crystallineand powdery polymers were obtained, which have melting points rangingfrom 166 TABLE 8 Polymerization conditions lolym- Results erizationYield Dose time (per- Thiocyanate Percent Acid anhydrlde Percent (racL)(hr.) cent) Km in] Methyl thiocyanate 6. 0 lroplonic anhydride. 1.0 1 760. 1. 7 D0 5. do 1.0 10 1 08 0.18 1.4 Ethyl thiocyanate 3. Aceticanhydride. 0.01 10 4 90 0.06 1. 6 D 3. ...d0 2.0 10 4 90 0.03 0.7 3.Propionic anhydride.. 1 0 10 4 99 0.05 1.4 1. Succinic anhydrlde. 2 0 104 99 0.07 2.2 1. Proplonic anhydride.- 0 1 10 2 65 0.32 2.0 1. do 1 0 102 70 0. 36 1. 7 Control:

Ethyl thioeyanate- 3. 0 10 4 98 0. 12 4. 3 Propionic anhydride. 1 0 10 238 1. 01 2.3 Benzyl thiocyanate- 1. 0 10 2 60 0. 43 3. 3

polymerization, the product was treated as in Example 45 to 172C. Thepolymerization conditions and results 9. A polymer of K 0.06, [11] 1.9was obtained in 98 percent yield.

The above experiment was repeated except that the mixture of tetraoxaneand the additives instead of only the tetraoxane was irradiated afterthe ampoule was sealed and before polymerization was carried out.Substantially the same result was obtained.

EXAMPLE 12 One gram of tetraoxane purified by sublimation inare shown inTable 8.

COMPARATIVE EXAMPLE 2 One gram of trioxane was put in a glass ampouleand thereto were added 1.0 percent propionic anhydride and 5.0 percentmethyl thiocyanate. Polymerization was carried out at 55C (which beingconsidered as the most preferable temperature for solid statepolymerization of trioxane) for 1 hour. No polymer was obtained.

EXAMPLE 14 TABLE 9 Polymerization conditions Results Yield Dose Temp.Time (per- Thiocyanete Percent Acid Anhydride Percent (rad.) 0.) (hr.)cent) Km [1 Methyl thiocyenate 5. 0 Propoinic anhydride 0. 06 1 50 0. 732. 3 D0 5.0 .do 0.05 10 90 1 83 0.46 2.1 3. 0 Acetic anhydride 1. 0 10 i4 99 0. 07 1. 4 1. 0 Succinlc anhydrld 2.0 1 99 0. 11 1. 8 o 1.0 do 1 010 106 4 99 0.09 2.2 Control:

Methyl thloeyenate 5. 0 10 90 1 71 0. 77 3. 5 Succinlc anhydride 1. 0 105 105 4 70 1. 21 2. 0

EXAMPLE 15 Ten( grams each of tetraoxane purified by sublimation was putin a glass ampoule, and theretowere added 0.05 percent of propionicanhydride and 5.0 percent of methyl thiocyanate, followed bypolymerization in liquid phase. After the polymerization, the productswere treated as in Example 13. The polymerization conditions and resultsare shown in Table 10.

TABLE 16 V H Polymerization conditions Results Yield Per- Temp. DoseTime (per- Solvent cent C.) (rad.) (hr.) cent) Km ['1] None 130 l 920.46 0.8 Cyelohexane 100 100 2 40 0.88 1.4 Do 100 100 10 1 58 0.50 1.0Control:

Cyclohexane 100 100 10 1 49 1.10 1.1

l In-source. 1 No anhydride employed.

EXAMPLE 16 EXAMPLE 17 One gram of tetraoxane purified by sublimation wasput in a glass ampoule, and 1.0% of propionic anhydride and 1.0 percentof methyl isothiocyanate were added thereto. The ampoule was sealed andpolymerization was carried out for 8 hours in l05C heating bath. Apolymer of [1 1.2 and K 0.32 was obtained in 25 percent yield.

When succinic anhydride was not used in the above experiment, the [1 andK of the resultant polymer were 1.7 and 0.97 respectively.

EXAMPLE 18 The starting mixture of Example 17 was irradiated with 2 X 10rad gamma rays and then polymerized in the same way as in Example 17. Apolymer of [n] 2.0 and K 0.19 was obtained in 88 percent yield.

What we claim is:

l. A process for preparing an oxymethylene polymer which comprisespolymerizing tetraoxane in the presence of at least one species ofanhydride of an organic carboxylic acid having no ethylenic double bondin an amount of about 10""l5% by weight of tetraoxane and at least onespecies of an organic sulfur polymerization initiator selected from thegroup consisting of a thiocyanate and an isothiocyanate in an amount ofabout l0 -l0% by weight of tetraoxane.

2. A process as set forth in c aim 1, in which said thiocyanate isselected from the group consisting of methyl thiocyanate, ethylthiocyanate, propyl thiocyanate, isopropyl thiocyanate, butylthiocyanate, isobutyl thiocyanate, amyl thiocyanate, isoamylthiocyanate, allyl thiocyanate, benzyl thiocyanate and phenylthiocyanate.

3. A process as set forth in claim 1, in which said isothiocyanate isselected from the group consisting of methyl isothiocyanate, ethylisothiocyanate, butyl isothiocyanate, isobutyl isothiocyanate, allylisothiocyanate, phenyl isothiocyanate and benzyl isothiocyanate.

4. The process of claim 1, wherein ionizing radiation is concomitantlyemployed.

5. The process of claim 1, wherein ultraviolet light is concomitantlyemployed.

1. A process for preparing an oxymethylene polymer which comprisespolymerizing tetraoxane in the presence of at least one species ofanhydride of an organic carboxylic acid having no ethylenic double bondin an amount of about 10 3- 15% by weight of tetraoxane and at least onespecies of an organic sulfur polymerization initiator selected from thegroup consisting of a thiocyanate and an isothiocyanate in an amount ofabout 10 4- 10% by weight of tetraoxane.
 2. A process as set forth inclaim 1, in which said thiocyanate is selected from the group consistingof methyl thiocyanate, ethyl thiocyanate, propyl thiocyanate, isopropylthiocyanate, butyl thiocyanate, isobutyl thiocyanate, amyl thiocyanate,isoamyl thiocyanate, allyl thiocyanate, benzyl thiocyanate and phenylthiocyanate.
 3. A process as set forth in claim 1, in which saidisothiocyanate is selected from the group consisting of methylisothiocyanate, ethyl isothiocyanate, butyl isothiocyanate, isobutylisothiocyanate, allyl isothiocyanate, phenyl isothiocyanate and benzylisothiocyanaTe.
 4. The process of claim 1, wherein ionizing radiation isconcomitantly employed.